With the recent concern over the depletion of oil resources, coal, the most abundant and prevalent of all fossil fuel resources and which once had become disfavoured in the competition with petroleum, is being newly considered as an oil substitute. However, as a very complex high molecular weight compound, coal contains not only carbon and hydrogen, the two primary components, but also signficant amounts of hetero atoms (oxygen, nitrogen and sulfur) as well as ash. Therefore, if it is simply burned, a large amount of air pollutants is generated, and the heating value of coal is not as high as oil. Furthermore, coal is more difficult to transport and store than oil.
To solve these problems inherent in coal, many processes have been proposed for liquefying coal to remove hetero atoms and ash, and obtain clean fuel oils and gases and various chemical materials having great commercial value. Typical processes include: (1) extracting coal with a solvent; (2) liquefying coal in the presence of hydrogen or a hydrogen donator; (3) liquefying and gasifying coal in the presence of hydrogen; (4) liquefying and gasifying coal in an inert gas.
A process is known for heating coal to obtain light oils and gas directly; in this method, a finely ground coal powder is injected into a hydrogen gas stream at high temperature and pressure for completing hydrogenation and thermal cracking of the coal within a very short period of several tens of milli-seconds to several minutes. More specifically, coal fines are injected into a hydrogen gas stream at a pressure of from 50 to 250 kg/cm.sup.2 G and a temperature of from 600.degree. to 1,200.degree. C. to heat the coal rapidly at a rate of from 10.sup.2 .degree. to 10.sup.3 .degree. C./sec for achieving both hydrogenation and thermal cracking of the coal. Methane, ethane, carbon dioxide, carbon monoxide, steam, hydrogen sulfide, and ammonia are formed as gaseous products; a gasoline fraction and heavy oils (aromatic compounds having 10 or more carbon atoms, and high-boiling tar) are formed as liquid products; and a solid product containing ash (referred to as "char") is obtained. But at low reaction temperatures, this process achieves only a low percent total conversion of coal into liquid or gas (the percent total conversion being defined as a hundred times the quotient of the number of carbon atoms in the total product as divided by the number of carbon atoms in the coal feed), and the principal product comprises aromatic compounds having 10 or more carbon atoms and heavy oils such as tar. If the reaction temperature is high, the percent total conversion is increased, but then, methane is the principal product, with a low percent conversion to light oils.
In an improved version of this method, coal particles as fine as 100 mesh or more are injected into a high-speed hydrogen gas stream to heat the coal at an increased rate of from 1,000.degree. to 10,000.degree. C./sec, and the reaction is completed at 700.degree. to 800.degree. C. within 2 to 10 seconds. By this improved method, the formation of methane is inhibited and yet the percent conversion into a gasoline fraction and other light oils is increased. However, even this improvement is unable to produce the gasoline fraction in a satisfactory high yield.
A method has been attempted wherein coal is hydrogenated and thermally cracked rapidly within a period of 20 milli-seconds to 2 seconds with a heating speed of 10.sup.4 .degree. C./sec or more at a reaction temperature of from 800.degree. to 1100.degree. C. and a pressure of from 35 to 100 kg/cm.sup.2 G (gauge pressure). If a very short period of 20 to 800 milli-seconds is used, conversion to a liquid product is as high as from 30 to 45 wt%, but conversion to the gasoline fraction is as low as from 3 to 8 wt%, and if the reaction time is prolonged, only the conversion to gases is increased, while the conversion to the gasoline fraction is decreased further.